<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <script type="text/javascript">
    //int num;
    //只是声明变量，没有赋值，现在的值是undefined
    var v;
    console.log(v);//undefined
    console.log(typeof(v));//undefined
    v = 3.14;
    console.log(v);//3.14
    console.log(typeof(v));//number
    v = 'js';
    console.log(v);//js
    console.log(typeof(v));//string
    v = true;
    console.log(v); //true
    console.log(typeof(v));//boolean
    v = null;
    console.log(v); //null
    console.log(typeof(v));//object
  </script>
</head>
<body>

</body>
</html>